1. Field of the Invention
The present invention relates generally to earth boring drill bits and more particularly to such bits which are assembled from two or more separately manufactured portions, one of which includes an external surface adapted to carry cutting elements and the other portion including a shank having a set of threads on one end thereof for connecting the drill bit to a string of drill pipe.
2. Description of the Related Art
Earth boring drill bits typically include a bit body upon which cutting elements are carried and a separate shank which is mounted on the bit body during the manufacturing process. The cutting elements may include natural or synthetic diamond cutters which are disposed on the bottom and sides of the bit body. The shank is coaxial with the bit body and includes a set of threads on an upper end thereof for connecting the drill bit to the drill string. One example of such drill bit is illustrated in U.S. Pat. No. 4,499,795 to Radtke for a method of drill bit manufacture.
In another known type of drill bit, an infiltratable powder, e.g., tungsten carbide, is formed in a mold around a steel inner core, referred to as blank to which a threaded shank is typically welded after infiltration. It is necessary to position forms in the mold which produce matrix coated ducts in the bit interior to provide flow paths for the drilling fluid to emerge on the face of the bit. After the core and powder are placed in the mold, it is placed in a resistance furnace, or alternatively an induction heater may be positioned adjacent the exterior of the mold, to heat and thereby infiltrate the powder. This is a time consuming process because the material in the mold presents a considerable mass and because the heat must be conducted from the exterior of the mold to the interior thereof. Cutters may be either integrated into the infiltrated matrix or brazed onto the matrix body after the matrix is infiltrated and the bit is removed from the mold. Complex ports which connect with nozzles on the face of the bit must be designed, built and placed into the mold prior to infiltration.
Prior art matrix bits use substantially solid cylindrical blanks which are heavy and which prevent use of matrix techniques for open bits such as that shown in U.S. Pat. No. 4,883,132 to Tibbitts.
In another prior art bit, U.S. Pat. No. 5,101,692 to Simpson discloses a drill bit manufacturing process in which separate portions are assembled to form a drill bit. One of the portions comprises an integrated bit shank and bit head core, referred to collectively as a bit shank. The other portion comprises a bit head in which the bit core is received. In Simpson, the bit head is investment cast from a steel alloy which is highly resistive to abrasive wear and fluid erosion such as a high content cobalt alloy like stellite. The bit shank is made from an easily machined steel and includes threads on both ends thereof, an upper set for connecting to a drill string and a lower set for threadably engaging corresponding threads formed in the bit head. After the head is cast and the shank machined, the same are threaded together and locked to one another by fusion bonding or by a mechanical lock.
According to the Simpson patent, the advantages of the manufacturing process described and claimed therein include providing a bit head and shank with different properties. The bit head is abrasive to wear and resistant to fluid erosion while the shank is easily machinable and has the capability of withstanding high stress or fatigue levels. Investment casting also provide very accurate surfaces for positioning cutter pockets on the bit head.
U.S. Pat. No. 5,150,636 to Hill discloses a rock drill bit and method of making same in which a head having a working face at one end and a recess at the other end having a cross-section corresponding in shape to the tip of a shank except that the recess is slightly smaller than the shank. In assembling the bit, the shank is cooled in a cryogenic gas to reduce its size and thereafter inserted into the head recess. When the components return to ambient temperature, a shrink fit is established between the head and the shank. As in Simpson, the shank and head are made of different materials with the head being made of material which is both tough and hard, such as hardened steels. Such material cannot be easily heated without damaging the head. If it is heated, it must be cooled very slowly to prevent the steel from becoming brittle.
It would be desirable to provide improved techniques for connecting a drill bit shell having an external surface for mounting cutting elements thereon to a complimentary bit core and shank. It would also be desirable to provide for utilizing matrix techniques for manufacturing a bit shell which is connectable to a complimentary integrated bit core and shank and which obviated the need for matrix coated internal ducts for porting drilling fluid to the face of the bit. Improved hydraulic supply and reduced erosion on the interior of the bit crown would also be desirable. It would be advantageous to reduce manufacturing time for drill bits and to eliminate the need for a conventional blank when manufacturing a matrix bit. An engineered, lightweight, blank which supports a matrix or other bit head and which is more open than prior art blanks would also be advantageous. It would be beneficial to provide for interchangeability of different external shells with a single bit core and shank and to provide a single design for a bit core and shank which is suitable for connection to a range of bit head shells having different external shapes or sizes.